1. Technical Field
The present invention generally relates to data telemetry systems for downhole sensors and other equipment and in particular to data telemetry over power cables. Still more particularly, the present invention relates to employing a modulated radio frequency carrier for data telemetry over power cables.
2. Description of the Related Art
Various data telemetry systems for returning measurements from sensors within a borehole or for transmitting commands to equipment within the borehole have been proposed and/or utilized. Several such systems employ the power cable transmitting three phase power downhole to an electrical submersible pump or other load device for transmitting the telemetry signals. Within these types of systems, generally the signaling arrangement either requires a ground reference for the return path or treats all three power conductor cables as a single conductor.
Systems which require a ground reference usually require an additional conductor for the return path. However, casing and tubing dimensions may not leave enough room for the additional conductor, the additional conductor adds to the cost of the system, and the additional conductor represents an additional point of possible failure for the system.
On the other hand, systems which treat the three phase power cable as a single conductor often cannot tolerate a ground referencexe2x80x94either intentional or inadvertentxe2x80x94within the power system. Thus, for example, if one phase or conductor of the power system should accidentally be shorted to ground, the downhole components which rely on the three-phase power (e.g., the pump) continue to operate while the telemetry system is disabled.
Moreover, systems employing the three phase power system for data telemetry are frequently limited to one receiving/transmitting device downhole, although it would often be useful to obtain data measurements at several locations within the borehole. In particular, data telemetry systems employing the three phase power cable powering a downhole motor and pump are generally positioned above the motor/pump assembly. Such measurements may be of limited value regarding the operation of the pump, which may extend for a significant distance down the borehole from the top of the motor/pump assembly.
In particular, when an electrical submersible pump (ESP) is employed, the motor/pump assembly is often as long as 60-70 feet, and may be as long as 90-100 feet. Measurements taken at the top of such a motor/pump assembly are not necessarily indicative of conditions at the bottom of the assembly. Measurements for a variety of conditions at the bottom of the motor and/or the bottom of the pump may be useful in monitoring or controlling operations, such as intake pressure and temperature, vibration, flow rate, revolutions per minute, winding temperature, discharge pressure and temperature, and xe2x80x9cwater cutxe2x80x9d (oil/water mixture).
It would be desirable, therefore, to provide a telemetry system employing three-phase power conductors for the data signals without requiring a return or ground reference conductor, but fault-tolerant with respect to unintentional grounding of one or two power phases. It would further be advantageous to provide a data telemetry system which allowed the use of multiple receiving and transmitting stations within the borehole.
Data or control signals are communicated over a three phase power cable supplying power from a surface location to a motor/pump assembly located within a wellbore utilizing modulated radio frequency signals. The radio frequency signals may be impressed on the power cable through physical taps to the power cable conductors or by reactive coupling to the power cable. The transmission frequency is selected from a range of frequencies which propagate through the motor windings and up the power cable with sufficient amplitude to be received and processed. The modulated RF signal may be transmitted concurrently with the three phase power on the power cable, and simultaneous bidirectional communications between the surface and downhole locations may be supported utilizing, for example, discrete frequencies for transmission in different directions. A network of RF transceivers or nodes may be situated at various locations along the wellbore and the motor/pump assembly to gather information about conditions at different points (e.g., below the motor/pump assembly, above the motor/pump assembly, and at the wellhead of a subsea borehole), with transmission on the power cable shared among the nodes through a spread spectrum and/or multiple access protocol.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.